Cellular networks for radio communication typically comprise multiple cells with radio coverage provided by base stations or other radio nodes, which thus can provide connections for mobile terminals, or User Equipments (UEs), when present in respective cells. In the following, the term “base station” will be used to generally represent a node providing radio coverage in a cell, which node is commonly referred to as a radio base station, base transceiver station, e-Node B, etc., depending on the technology and terminology used. The term “terminal” will also be used for short in this description to represent any wireless or mobile terminal or device capable of radio communication with a base station. It should be further noted that this description pertains to any technologies and communication standards where cell evaluation can be employed, such as Global System for Mobile communication (GSM), Universal Mobile Telecommunications System (UMTS), Long Term Evolution (LTE), etc.
A cellular network is typically divided into so-called “tracking areas”, sometimes also termed “registration areas”, “location areas” or “paging areas”. Such tracking areas are used to enable paging of terminals based on e.g. tracking area update messages transmitted regularly from the terminals to the network as they move from one tracking area to another. In this way, the network can page a terminal for an incoming call or session by transmitting a paging message only in the cells of the terminal's latest reported tracking area. Thus, an idle terminal needs to be served by a base station in an appropriate cell in order to detect such a paging message, among other things.
When a terminal is present in an area of cells covered by different base stations, it selects one of the cells to be its “serving cell” by measuring cell-specific Reference Signals (RSs) being constantly transmitted, typically with fixed power, in the cells from respective base stations. The cell-specific reference signal may be denoted depending on the standard used, such as “RS signal” or “common pilot channel”. In some examples, LTE uses a Cell Specific Reference signal (CRS), Wideband Code Division Multiple Access (WCDMA) uses a Common Pilot Channel (CPICH) and GSM uses a Broadcast Control Channel (BCCH) for estimating signal quality. The terminals are thus configured to receive and measure such reference signals from different base stations, and to evaluate the cells based on an estimated quality of the received reference signals in order to select a serving cell to “camp” on.
The signal quality can be estimated in different ways, e.g. by measuring the received power or strength of the reference signal, or by measuring the power-to-interference ratio of the reference signal. The signal quality estimate of the cell obtained in this way will be referred to as Reference Signal Quality (RSQ) in the following, regardless of how the signal quality is estimated in practice. The cell having the “best”, i.e. highest, RSQ is then typically selected by the terminal as the serving cell, which the terminal can use for receiving network information, listening for paging messages, sending tracking area update messages, as well as for establishing a connection for a communication session when required.
The process of evaluating cells for selecting a cell as suitable for being the terminal's serving cell is commonly referred to as “cell selection”, while the process of selecting a new cell to replace a current cell as the terminal's serving cell is commonly referred to as “cell re-selection”. The terminals are configured to frequently perform cell evaluation when not engaged in a communication session, which is sometimes called that the terminals are in the “idle state”. A terminal may thus change its serving cell, i.e. perform cell re-selection, when the estimated RSQ from a new cell is sufficiently higher than the estimated RSQ from the current serving cell.
The decision whether to change the serving cell or not is taken by the terminal based solely on the above signal measurements. Typically, it also depends on a predefined cell evaluation condition. For example, the cell evaluation condition may dictate that re-selection from a current serving cell to a new cell should be made when the RSQ of the new cell exceeds the RSQ of the current serving cell with a certain amount “A”, or when the RSQ of the new cell exceeds a certain threshold “T”. The above parameters A and T may be configurable, e.g. to avoid unwanted re-selection oscillations while still ensuring that a proper cell is selected for correct and efficient signal reception and radio performance. A hysteresis may also be employed such that a new cell re-selection cannot occur until a certain time has elapsed since the latest cell re-selection.
However, it has been recognized as a problem that the RSQ is not always a very accurate indicator of the radio performance in the cell but can actually be rather misleading sometimes, such as when the cells have different abilities for serving terminals in terms of data communication and accessibility.
Even though it may be possible to deal with this problem, at least partly, by configuring the above parameters A and T to compensate for any such differences between cells, considerable efforts and knowledge are required from the network operator to enable useful and comparable measurements of the RSQ for evaluation of the cells. Moreover, the parameters A and T may need to be configured dynamically due to changing conditions and must also be communicated to all terminals for them to apply the parameters properly when evaluating their signal measurements.